The transformation shown below can be accomplished in TWO reaction steps. From the following list, select the two reagents (or sets of reagents) which would most effectively bring about this conversion. Reagents:

1. Conc H2SO4/heat
2. CN-
3. KMnO4
4. (i) CH3MgI (ii) H3O+
5. PBr3
6. (i) BH3 (ii) H2O2
7. CH3CH2O-/CH3CH2OH
8. H2O/H+
9. OH-/H2O
10. (i) O3 (ii) Zn
11. NaBH4
12. HCl
13. KI
14. Na
15. CH3CH2Br

Please enter the reagent number (from the above list) for each of the two steps in the following synthesis:
Step 1:
Step 2:

CH3-CH2-CHO (Step 1)-> (Step 2)-> CH3CH2COCH3

To determine the reagents needed for each step in the given transformation, let's analyze the functional groups and possible reactions.

Step 1: Conversion of an aldehyde (CH3-CH2-CHO) to a ketone (CH3CH2COCH3)
- In this step, we need a reaction that converts an aldehyde into a ketone. This is commonly done by using a reducing agent to add a hydrogen atom to the carbonyl group, transforming it into a hydroxyl group.

Reagents that could accomplish this conversion are:
- 4. (i) CH3MgI (ii) H3O+: This is a Grignard reaction, where the Grignard reagent (CH3MgI) adds to the carbonyl group, forming an alcohol. The subsequent addition of acid (H3O+) would convert the alcohol to the corresponding ketone.
- 14. Na: Sodium metal in the presence of an alcohol (H2O) can also reduce the aldehyde to an alcohol. The resulting alcohol can then be oxidized to a ketone using a mild oxidizing agent (e.g., Jones reagent or PCC).

Step 2: Conversion of the alcohol to an acetal (CH3CH2COCH3)
- To convert the alcohol to an acetal, we need a reaction that reacts with the hydroxyl group, forming a new carbon-oxygen bond and eliminating water.

Reagents that could accomplish this conversion are:
- 6. (i) BH3 (ii) H2O2: This is a hydroboration-oxidation reaction. BH3 reacts with the alcohol to form an intermediate, while the subsequent addition of H2O2 converts it into the desired acetal.
- 7. CH3CH2O-/CH3CH2OH: A strong base like CH3CH2O- (sodium ethoxide) can deprotonate the alcohol, creating an alkoxide nucleophile. This nucleophile can then attack another equivalent of the alcohol, forming an acetal.

So, the reagent numbers for each step are:
Step 1: 4. (i) CH3MgI (ii) H3O+
Step 2: 6. (i) BH3 (ii) H2O2